Combustion of elemental sulfur in air or in an oxygen-containing gas stream is a convenient method for generating sulfur dioxide. However, combustion of liquid sulfur effected within a body of the same results in a stream of gaseous product heavily laden with vaporized elemental sulfur. High elemental sulfur content of such gases makes them unsuitable for bleaching or other processes wherein substantially sulfur-free sulfur dioxide is desired or required. Moreover, the vaporized sulfur tends to deposit as liquid or solid upon surfaces of equipment, creating or aggravating operational and design difficulties.
U.S. Pat. No. 2,726,933 to Merriam et al. describes a method for making sulfur dioxide-containing gas streams substantially free of oxygen and sulfur which involves contacting an oxygen-containing gas stream with liquid sulfur maintained at auto-ignition temperature (at about 500.degree. F. or above) within a reaction zone comprising a pool of molten sulfur. The resultant gases comprising sulfur dioxide and vaporous sulfur are contacted further with liquid sulfur having temperature below auto-ignition, for time and under conditions adequate to substantially reduce the temperature of those gases to obtain a sulfur dioxide-containing gas stream substantially free of oxygen and free sulfur. Merriam et al. found that if in the second step the gases are cooled to temperature below minimum auto-ignition temperature of the sulfur, by about 200.degree. F. or more, then their sulfur content may be nil for practical purposes. The sulfur dioxide-containing gas stream obtained by the method of Merriam et al. is sufficiently free of elemental sulfur for use in bleaching or reduction of paper or other cellulose material, or for exothermic reaction with hydrogen sulfide to produce elemental sulfur.
However, we have found that when a sulfur dioxide-containing gas stream made in accordance with Merriam et al.'s method is used for making sodium sulfite by contacting an aqueous sodium carbonate or hydroxide solution with the sulfur dioxide-containing gas stream, then the sodium sulfite product always contains as impurity elemental sulfur or significant amounts of sodium thiosulfate as a result of reaction of initially formed sodium sulfite with traces of residual elemental sulfur contained in the gas stream. Further investigation showed that Merriam et al.'s method is incapable of consistently producing a sulfur dioxide-containing gas stream containing less than about 0.0002 lb. of sulfur per cubic foot of gas, measured at standard conditions. While that sulfur level is sufficiently low for most applications wherein a sulfur dioxide-containing gas stream is required having low oxygen and sulfur contents, that already low sulfur content is still not sufficiently low if the sulfur dioxide is to be used for making photographic grade sodium sulfite and sodium metabisulfite.
Accordingly, it is an object of the present invention to improve the process disclosed by Merriam et al. to further reduce sulfur content of the sulfur dioxide-containing gas stream made in accordance with that process.